1. Field of the Invention
This invention relates to a method for determining liquefaction potential of soils, particularly a method for determining liquefaction potential, in-situ of cohesionless soils.
2. Description of the Prior Art
Soil liquefaction results from increases in soil porewater pressure induced by transient or repeated ground motions. Porewater pressure increases may be induced by earthquakes, explosions, impacts and ocean waves. Soil liquefaction occurs in water-saturated, cohesionless soils and causes a loss of soil strength that may result in the settlement of buildings, landslides, the failure of earth dams and pipelines, or other hazards. Liquefaction of sands and silts has been reported in almost all large earthquakes around the world. For example, an earthquake in 1964 caused more than one billion dollars in damages in Niigata, Japan. The Great Alaskan Earthquake in 1964 destroyed large sections of Valdez and Anchorage, and failed more than 250 bridges. The San Fernando, Calif., earthquake of 1971 resulted in the liquefaction induced failure of the Lower Van Norman Dam. Most of the damage that occurred during these earthquakes was due to widespread soil liquefaction.
For existing and planned structures, e.g., large mine tailing impoundments, earth fill dams, nuclear power plants and offshore structures, the liquefaction tendencies of a site should be studied so that preventative steps can be taken where possible.
Prior methods for evaluating liquefaction potential follow two basic approaches, laboratory tests and in-situ test procedures. The laboratory approach requires undisturbed samples. At the present time, there is not a method for obtaining undisturbed samples which does not alter the in-situ void ratio, structure, or stress state during removal. There are also questions on which laboratory test is more representative of actual field conditions. As for the other approach, four in-situ tests are being used to determine liquefaction potential. These are (1) the Standard Penetration Test (SPT), (2) the Cone Penetration Test (CPT), (3) The Piezocone Penetration Test (PCPT) and (4) Seismic Wave Tests (SWT).
The Standard Penetration Test (SPT) approach is based on an empirical correlation between the number of blows and the occurrence or nonoccurrence of liquefaction at sites subjected to past earthquakes. The SPT data is sensitive to technique and can vary by more than 50% among reputable drillers. The Cone Penetration Test (CPT) has several advantages over the SPT, but like the SPT test, empirical correlations between penetration resistance and liquefaction potential are used. The piezocone penetration test (PCPT) uses the CPT with a porewater transducer located on or behind the cone. There is some disagreement on the location of the pressure transducer and what is measured. A major disadvantage is that the soil displacement caused by the penetration of the cone and vertical stress increases caused by the advancement of the cone, influences the porewater pressure response. Seismic Wave Tests (SWT) are based on an empirical correlation between the velocity of shear waves and the occurrence or nonoccurence of liquefaction.
Samples of the above penetration tests are disclosed in e.g., in U.S. Pat. No. 4,453,401 to Sidey (1984) and U.S. Pat. No. 4,594,899 to Henkel et al. (1976), which disclosures are subject to the drawbacks noted above, relative to the SPT approach and CPT approach.
There have also been attempts to measure soil liquefaction potential by employing a shear vane test. In this approach a vane assembly, having a plurality of angularly spaced blades, is mounted to a drilling shaft, with torque sensors mounted either on the blades or the drilling shaft. The so-mounted vane assembly is lowered on the drilling shaft, into soil, rotated and torque measurements taken. Subsequently, mathematical calculations based on the respective torque measurements, result in an approximation of soil shear strength.
In another example of the prior art, a laboratory vane apparatus has been constructed wherein 4 blades at right angles are mounted on a rotational shaft with an aperture cut through one of the vanes which communicates with the interior of said shaft and to the upper portion thereof and thence to a hypodermic needle, for registering changes in pore pressure at the edge of such vane. The vane apparatus was lowered into a container of cohesionless soil such as silt or sand. The vane blades were rotated in the sand to shear same and describe a cylinder having the same dimensions as the (outer edges of the) vane blades. The blades were rotated slowly and porewater pressure fluctuations noted on the calibrations of the hypodermic needle. A number of tests were accordingly run in dense sand (dilative condition) and loose sand (contractive condition) and the porewater pressure noted. It was found that in the dense sand one would obtain negative porewater pressure readings and that in loose sand, one would obtain positive porewater pressure readings. The conclusion of such studies was that torque readings were distorted in cohesionless soils so as to indicate, in dense sand, an inflated stress reading while in loose sand, a deflated or reduced stress reading relative to shear strength was indicated. That is, the study concluded that one could not use rotating vane shear tests to obtain accurate shear strength calculations in cohesionless soil but only in cohesive soil such as clay. The study thus recommends against the use of a rotational vane apparatus for making shear strength related measurements in a cohesionless soil, see Wilson, N. E., 1963 "Laboratory Vane Shear Tests and The Influence of Pore-Water Stresses," ASTM, Special Technical Publication, No. 361, pp 377-388. For a related article see ASTM D 2573, 1972, re-approved 1978, "Standard Test Method for Field Vane Shear in Cohesive Soil," 1987, Annual Book of ASTM Standards, Vol 04.08, Phila, Pa., pp. 424-427.
Such article covers the field vane test in soft saturated cohesive soils with guide lines for conducting the test and also a formula for calculating shear strength of the soil from the torque applied to the vane apparatus.
Indications are from the above two articles, particularly the former, that one would not use a vane assembly for tests in cohesionless soils.
There has thus been no satisfactory test for measuring shear strength and liquefaction potential for water-saturated, cohesionless soil deposits. And there is a need and market for a procedure for such soil testing in which such soil is not locally compressed or otherwise deformed so as to introduce significant error into such measurements, which are important for e.g., site and foundation response studies.
There has now been discovered a method for obtaining the above needed data accurately, to determine therefrom the liquefaction potential and shear strength of cohesionless soils.